New Conversion of Solar Energy to Electricity by Natural Dye Sensitization 1 2 for merge

Conversion of Solar Energy to Electricity by NaturalDye-Sensitization

C.I.F. Attanayake 1. B.A.J.K. Premachandra 1. A.A.P. de Alwis1. and

G.K.R. Senadheera 2.

1. Department of Chemical and Process Engineering, University of Moratuwa, Moratuwa, Sri Lanka (e-mail : [email protected], [email protected], [email protected])

2. Institute of Fundamental Studies, Hantana Road, Kandy (e-mail: [email protected]) ,Open University of Sri Lanka , Kandy Regional Centre, Kandy, Sri Lanka

Abstract

Preliminary investigations on the identification of naturalpigments in the dye-sensitization of nanocrystalline n-typeTiO2 were carried out. Fresh extracts of Mangoostein, Rambutan,Mango, Tomato, Carrot, King coconut, Pumpkin, Red Banana,Beetroot, Turmeric, Venivel, Orange, Grape, Spinach, Wattakka,Ginger etc were employed as sensitizers in thin layer sandwichtype photo electrochemical dye – sensitized solar cells(DSSC's).

After electrical and electronic analysis, it was observed thatmany useful dyes which could be extracted from naturalproducts by simple procedure could be used as photosensitizers for DSSC's. It was also observed that dye extractsof Turmeric and Mangoostein yielded better results.

The current-voltage curves obtained with solar cells employingthe photo anode with TiO2 sensitized by different dyes wereobserved. The values of short circuit current density (Jsc),open circuit voltage (Voc), fill factor (ff), and efficiency(η) obtained for solar cells employing photo anodes with TiO2

sensitized with different fruit / vegetable extracts werenoted . The dye extracts of Turmeric root and Mangoosteinfruit were found to be superior to those obtained from otherdyes , and were Jsc = 0.540 mAcm-2 and 0.444 mAcm-2, Voc =599.1 mV and 565.2 mV, ff = 69.03 % and 65.66 % , η = 0.223 %and 0.165 % respectively.

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This work done incorporates the foundation on which researchcould be done to develop low-cost, high efficiency solarenergy to electricity conversion units for use in countrieslike Sri Lanka.

1. Introduction

Technological achievements in the clean energy systeminfrastructure are a fundamental issue for worldwide economyand environmental improvements. Therefore, in this 21st

century, energy based non-renewable sources has to beconverted into new energy systems by incorporating noveltechnologies derived from advancements in science [(O'Reganand Grätzel, 1991)]. Among several new energy technologies,Die-Sensitized Solar Cells (DSSC's) are one of the mostpromising new energy generation systems for photovoltaictechnology. It has emerged as one of renewable energy sourcesas a result of exploiting several new concepts and materials,such as nanotechnology and molecular devices. Even though thefirst dye sensitization of semiconductors was reported byVogel in 1873, where Silver halide emulsions were sensitizedby dyes to produce black and white photographic films, the useof dye sensitization in photovoltaic’s had been achievedlittle noticeable result until a break through at the early1990's by Gratzel's group. They developed a DSSC consisting ofTiO2 electrode sensitized with Ruthenium (II) complex dye,organic liquid electrolyte with iodine/iodide red ox coupleand Platinum deposited counter electrode. The solar energy toelectricity conversion efficiencies were reported on high as7.1% in 1991 and 10% and 11% in 2008 (Grätzel, 2003). In thesedevices a monolayer of the dye is directly attached to thesemiconductor surface via carboxyl group, which could realizean efficient injection of charge carriers from photo exciteddye to semiconductor. However this sensitization of TiO2 forsolar applications requires not only efficient but also stableand inexpensive sensitizers. So far, several organic dyes andorganic metal complexes have been employed to sensitizenanocrystalline TiO2 semiconductors and one of the most

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efficient sensitizer is transition metal coordination compound(Ruthenium polypyridyl complex). This is because the complexhas intense charge – transfer (CT) absorption in the wholevisible range, long excited lifetime and highly efficientmetal-to-ligand charge transfer (MLCT).

However, other organic dyes, such as phythalocyanine, cyaninedyes, xanthalene dyes, coumarin dyes and so on usually performpoorly in DSSC's because of weak binding energy with TiO2 filmand low charge transfer absorption in the whole visible range,but these dyes are very cheap and can be prepared easily,compared to Ruthenium polypyridyl complexes. On the otherhand, in nature, the fruit, flower, root and leaf of plantsshow various colours from red to purple and contain variousnatural dyes which can be extracted by simple procedure.Therefore, it has been emphasized by many researchers toobtain useful dyes as photo sensitizers for DSSC's fromnatural products, because of the simple preparationtechniques, widely available sources, and low cost (Smested1998, Hao et al 2006). Due to these reasons the importance ofwork done by the authors to develop low cost solar energy toelectricity conversion units in principle is emphasized.

2. Methodology

2.1 Extraction of dyes

The extracts of dyes from various fruits and vegetables wereobtained from fresh fruits and vegetables. The clean fruitsand vegetables were crushed and added to ethanol (Merck). Whennecessary, the mixtures were centrifuged and all solutionswere protected from direct light exposure.

2.2 Preparation of nanocrystalline TiO2 films

A TiO2 paste was prepared by blending 3 grams powder of TiO2

(P-25, Degussa) and 0.1 ml of Triton X 100 in an agate mortar,then the mixture was ground for 30 min, finally 10ml ofethanol was slowly added whilst grinding continuously for theanother 30 min. Above pastes were then applied on Fluorine-

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doped Tin oxide coated transparent conducting glass substrateby well known Doctor Blade method to obtain approx 10micrometer thick TiO2 film. Films were then heat treated at5500C for 30 min. and cooled down to room temperature. Thenthey were immersed separately in alcoholic dye solutions for12 hours.

2.3 Fabrication and characterization of solar cells

Photo electrochemical solar cells were then fabricated bysandwiching a Platinum sputtered conducting Tin oxide (CTO)glass plate with the dyed TiO2 films. A red ox electrolytecontaining / redox couple was then introduced to the

solar cells. I-V characteristics of the solar cells at 100mWcm-

2 (AM 1.5) were measured using a home-made I-V measuring set upcoupled with Keithley 2000 Multimeter with a Potentiostat viaa computer controlled software available at the Institute ofFundamental Studies (IFS), Kandy. Xenon 500 lamp was also usedwith AM 1.5 filters to obtain simulated sunlight with theintensity of 100 mWcm-2. The intensity of the light wascalibrated using an EKO Pyronometer and Silicon photodiode.The Absorption Spectra were also obtained with the UV 2450SHIMADZU UV-VIS Spectrophotometer available at the IFS.

3. Results and Discussion

3.1 Absorption Spectra obtained from various fruits andvegetables

Figure 1 at Appendix depicts the absorption spectra ofethanolic dye solution. It can be seen that, the dye solutionobtained from the Mangoostein absorb more in the red side ofthe spectrum than the other dyes.

The current voltage curves obtained with a few solar cellsmeasured with dyes initially employing the photo anode withTiO2 sensitized by different dyes is presented in Figure 2 atAppendix. Table 1 presents the values of short circuit currentdensity (Jsc), open circuit voltage (Voc), fill factor (ff),

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and efficiency () obtained for solar cells employing photoanodes with TiO2 sensitized with different fruit / vegetableextracts. Fill factor values from 0.30 to 0.64 were obtainedwith these dyes. The average efficiency values obtained forcells having the dye from Mangoostein fruit is superior tothose obtained from other dyes. This could be due to betterinteraction between the dye molecules and the surface of TiO2.

Subsequent analysis and evaluation of further samples ofnatural dyes extracted from a cross section of plants grown inSri Lanka is presented at Table 2 in Appendix. From this Tableit is observed that extracts of Turmeric rhizome root yieldedbetter results amongst these dyes tested.

Even though the efficiency values obtained in this study arenot significant with the values obtained in the system withRuthenium complexes, the straight forward preparation of photoanodes with semiconductor oxides sensitized by natural dyesstill enables, a cheaper and easy environmentally friendlyproduction of solar cells (Hao et al 2006). Further itprovides an interesting alternative to commonly used syntheticdyes. Therefore, investigations are being carried out insearching for an efficient natural dye which can havepotential use in these DSSC's.

This work done incorporate the foundation on which research isto be done to develop low cost, high efficiency solar energyto electricity conversion units for use especially in ruralareas of Sri Lanka where access to the national gridelectricity supply is not available. This would also enable toalleviate poverty and to improve living standards amongstrural communities.

4. Research Priorities

The main research concerns for dye- sensitized solar cellsare the reduction of material degradation that leads to poordevice longevity, affordable encapsulation methods to protect

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against environmental degradation , alternatives to dyes assensitizer , and development of solid electrolytes to avoidleakage problems .

5. Conclusions

This paper describes an investigation of natural dyes ofplants grown in Sri Lanka as natural photosensitizes. Extractsof Turmeric root and Mangoostein fruit rind have achieved fillfactors, of nearly 70% and 65% respectively, and solar energyconversion efficiencies of 0.223% and 0.165% respectively.

Natural dyes based solar cells appear to be limited by low Vocand Isc. Finding different additives for improving them mightresult in larger conversion efficiencies. Although naturaldyes are still below the present requirements, the results areencouraging and may boost additional studies oriented to thesearch of new natural dye sensitizers.

Acknowledgements

Provision of laboratory facilities to carry out the above -mentioned experiments on DSSC's out the above mentionedexperiments at the Institute of Fundamental Studies (IFS),Hantana, Sri Lanka under the guidance of Professor G.K.R.Senadheera is gratefully acknowledged. Also the support adviceand guidance for this research work provided by ProfessorA.A.P. De Alwis, Dr. B.A.J.K. Premachandra of the Universityof Moratuwa are also acknowledged with gratitude.

References

1. O Regan, B., Gratzel, M., 1991. A low-cost, high-efficiency solar cell based on dye-sensitized colloidalTiO2 films. Nature 353, 737 – 740.

2. Grätzel, M., 2001. Photo electrochemical cells. Nature414, 338-344.

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0 100 200 300 400 500 600 700Voltage (m V)

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-2 )

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(h)(i)(f)

350 400 450 500 550 600 650 700 750 800W ave length (nm )

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alize

d Ab

sorb

ance

ad hbc

g

i

f

e

3. Smested, G.P., 1998. Education and solar conversion:Demonstrating electron transfer. Solar Energy Materials & Solar Cells 55, 157-178.

4. Grätzel, M., 2003. Dye-sensitized solar cells. Journal ofPhotochemistry and Photobiology C: Photochemistry Reviews(4), 145-153.

5. Hao, S., Wu, J., Huang, Y., Lin, J., 2006. Natural dyes asphoto sensitizers for dye-sensitized solar cells. Sol.Energy 80, 209-214.

6. Hasan,M.S., Roy,S., Corkish,R., 2010. Sustainable Energyin Asia and the Pacific:Emerging Technologies andResearch Priorities in Solar Energy, 28- 33.

APPENDIX

Figure 1. Absorption spectra of dye solutions extracted from(a) Mangoostien (b) Rabutan (c) Mango (d) Tomato (e) Carrot(f) King coconut (g) Pumpkin (h) Red Banana (i) Beetroot

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Figure 2. Current-Voltage characteristics of pigmentsensitized solar cells (a) Mangoostien (b) Rabutan (c) Mango(d) Tomato (e) Carrot (f) King coconut (g) Pumpkin (h) RedBanana (i) Beetroot

Table 1 Characteristics of a few solar cells measuredinitially with different natural dye extracts.

Summary of Results

Voc(mV)

Jsc(mAcm-2)

ff% %

Mangoostien (a) 556 0.432 64 0.153Rabutan (b) 504 0.312 46 0.072Mango (c) 567 0.210 43 0.050Tomato (d) 577 0.194 44 0.052Carrot (e) 641 0.154 35 0.035King coconut (f) 256 0.150 30 0.011Pumpkin (g) 622 0.076 36 0.017Red Banana (h) 473 0.034 52 0.008Beetroot (i) 256 0.018 37 0.002

Table 2 Summary of Photo Electro Chemical Parameters of DCCC'susing natural dyes from a cross section of plants grown in SriLanka (with Decreasing Efficiency Values)

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Ser.No.

Dye VOC(mv)

ISC(mA)

ID(mA/cm2

)

FillFactor(%)

Efficiency(%)

Solvent

01 Turmeric 599.1

0.135

0.540 69.03

0.223 Ethanol

02 Mangoostien 565.2

0.111

0.444 65.66

0.165 Ethanol

03 Mangoostien-skin

635 0.087

0.348 69.41

0.153 Ethanol

04 Mangoostien 563.6

0.206

0.412 63.63

0.148 Ethanol

05 Mangoostien-skin

567.5

0.094

0.376 57.38

0.142 Ethanol

06 Mangoostien-skin

593.4

0.081

0.324 68.6 0.131 Ethanol

07 Mangoostien-pulp

631 0.065

0.260 67.73

0.111 Ethanol

08 Venivel 529.9

0.079

0.316 57.59

0.097 Ethanol

09 Orange 627.5

0.05

0.200 73.1 0.091 Ethanol

10 Mangoostien + Orange + Grapes

600.8

0.042

0.168 71.91

0.073 Ethanol

11 Rambutan 504.1

0.156

0.312 45.53

0.072 Ethanol

12 Mangoostien + Orange + Grapes

594.7

0.045

0.180 59.35

0.063 acetonitrilewith tert

13 Orange 624.9

0.039

0.156 64.6 0.063 acetonitrilewithtert

14 Grapes 498 0.049

0.196 60.28

0.058 acetonitrilewithtert

15 Rambutan 493.9

0.047

0.188 61.67

0.057 Ethanol

16 Orange 558 0.0 0.228 73.8 0.057 Ethanol9

.3 34 617 Rathadun 427

.20.069

0.276 74.39

0.056 Ethanol

18 Venivalgata 538.4

0.039

0.156 67.37

0.056 Ethanol

19 Orange 619.4

0.039

0.156 55.25

0.053 acetonitrilewithtert

20 Mango 584.4

0.093

0.186 46.92

0.051 Ethanol

21 Mangoostien-skin

665.5

0.034

0.136 57.03

0.051 acetonitrilewithtert

22 Grapes 557.8

0.035

0.140 62.86

0.049 Ethanol

23 Spinach 546.5

0.032

0.128 66.43

0.047 Ethanol

24 Bulu 493.7

0.041

0.164 74.44

0.038 Ethanol

25 Mangoostien + Orange + Grapes

608.9

0.026

0.104 56.82

0.036 acetonitrilewithtert

26 Carrot 641.2

0.077

0.154 35.4 0.035 Ethanol

27 Grapes 538.2

0.024

0.096 61.32

0.032 acetonitrilewith tert

28 Grapes 566.8

0.015

0.060 72.22

0.025 Ethanol

29 Rathadun 415 0.026

0.104 48.58

0.021 Ethanol

30 Beli Flower 490.2

0.019

0.076 49.72

0.018 Ethanol

31 Nelum Seed 497.1

0.022

0.088 39.04

0.017 Ethanol

32 Pumpkin 622.2

0.038

0.076 36.28

0.017 Ethanol

33 Kothala 450 0.0 0.088 38.8 0.016 Ethanol10

Himbutu .1 22 734 Tomato 507

.50.016

0.032 79.69

0.013 Ethanol

35 Aralu 468.9

0.026

0.104 26.83

0.013 Ethanol

36 Purple Makaral 380.7

0.025

0.100 34.13

0.013 Ethanol

37 Walmadata 520.7

0.084

0.336 68.54

0.012 Ethanol

38 Ginger 489.5

0.013

0.052 40.72

0.011 Ethanol

39 King CoconutNut Husk

256.7

0.073

0.146 30.48

0.011 Ethanol

40 Banana 599.2

0.021

0.084 78.77

0.004 Ethanol

41 Red Banana 476 0.016

0.016 48.62

0.004 Ethanol

42 Beetroot 252.7

0.009

0.018 37.54

0.002 Ethanol

43 F.Berry 383.3

0.006

0.024 26.3 0.002 Ethanol

44 B.Onion 331.8

0.002

0.008 32.87

0.001 Ethanol

45 Beli 194.3

0.003

0.012 23.01

0.001 Ethanol

46 PurpleMaakaral

165.1

0.003

0.012 35.56

0.001 Ethanol

47 Adesia Fruit 182.1

0.002

0.008 29.2 0.001 Ethanol

48 Rata Lovi 164.4

0.001

0.004 36.1 0.001 Ethanol

49 Wadukuda Fruit 178.2

0.003

0.012 34.9 0.001 Ethanol

50 Goraka 323.3

0.002

0.008 27.91

0.001 Ethanol

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